1. Field of the Invention
The present invention relates in general to microwave circuits, and more particularly, to the packaging of negative resistance diodes and the circuits employed therewith.
2. Description of Related Art
For more than a decade, there has been substantial interest in development of solid state microwave and millimeter wave diodes which are utilized in a variety of power generation, control and signal processing functions. For example, a negative resistance diode, such as an IMPATT diode, is often employed in an oscillator or an amplifier to convert DC power to radio frequency power. IMPATT diodes are often employed in radio frequency applications where a very high frequency, relatively high conversion efficiency, and solid state reliability are required. IMPATT diodes can be manufactured in great quantities and at low cost. However, a key to ringing every milliwatt of power from such diodes lies in the packaging arrangement for the diode which must provide mechanical support for the diode, input and output circuitry for the diode, and impedance matching between the diode and the RF circuit in which the diode is operated, all of which must be accomplished in the smallest package possible without sacrificing reliability or efficiency.
In a conventional diode packaging arrangement for IMPATT amplifiers, for example, an IMPATT diode chip is mounted on a thermally and electrically conductive cylindrical copper heat sink. A ceramic ring is mounted on the heat sink encircling the diode chip, and gold bonding straps are soldered to the top of the ceramic ring and also to the diode chip, respectively. A thin metal disc is placed over the bonding straps and soldered thereto and serves as the cap to the diode packaging arrangement hermetically sealing the diode. The heat sink, diode chip, ceramic ring and cap form the basic diode package and this assembly is inserted into the rf circuit through a hole in a housing base and followed by a locking screw which holds the cylindrical heat sink in place. A coaxial transmission line structure sits over the diode. This coaxial transmission line structure generally includes several adjacently stacked outer conductors which form a central passageway of varying diameters for an inner conductor disposed therein. The outer and inner conductors provide in combination a multi-section coaxial transmission line for impedance matching. One end of the inner conductor is coaxially disposed on the diode cap and makes electrical contact thereto to provide a DC bias to the IMPATT diode. The cylindrical heat sink forms the ground electrode for the diode.
The multi-section coaxial structure and the IMPATT device package are generally the more difficult elements to align in an IMPATT amplifier or oscillator assembly. Several problems are generally associated with fabricating the above-described arrangement and providing the desired impedance matching between the IMPATT diode chip and the output waveguide. In order to provide optimal impedance match of the diode with the circuit, the IMPATT device package must be coaxially aligned with the inner center conductor and with the outer conductors. This is especially critical for the first closely spaced outer conductor of the multi-section coaxial structure. The width of the annular gap between the outer conductor and center inner conductor may be as little as about one mil. Typically, achieving and maintaining the required concentricity of these parts is difficult to accomplish requiring high cost precision machining and precise placement of the respective parts. The center conductor bias pin must also maintain a close sliding fit within a bias choke which is typically employed to tune the circuit; even small play of the bias pin can destroy the concentricity of the bias pin in the close-fitting coaxial section. Furthermore, environmental conditions such as temperature cycling, vibrations and shock may adversely affect alignment of the individual parts.
Additionally, there may be side-to-side movement when the diode is inserted into the circuit with the tightening of the locking screw. Ultimately, once the diode is assembled on the heat sink and the heat sink inserted into the RF circuit, proper alignment thereof cannot be inspected or easily corrected. Also problematic in the conventional configuration is the electrical contact made between the end of the bias pin and the cap of the diode package. The contact between these two parts is dry, no soldering or welding, resulting in I.sup.2 R type RF losses. A circuit configuration which reduces these RF losses would be a great advancement. Additionally, a circuit arrangement is needed which mitigates the possibility of relative movement of the coaxial transmission line section and the diode.